In recent years, systems in which digital contents, such as music, video, images, and games, are distributed via the Internet or a digital broadcast and then used have reached the implementation stage.
As can be understood from the patent documents #1 to #3 listed below, in a conventional content using system, a content that has been distributed via a broadcast is bound to a network so that the content can be shared within the network and used. Here, the “binding” of a content to a network refers to a condition where only authorized terminals on the network are able to use the content. Putting this another way, if a content is bound to the network, even if a different, non-authorized terminal is connected to the network, such terminal will not be able to use the content. Additionally, if a content that has been bound to a network is moved to another network, terminals on this other network will not be able to use the content.
As one example, when a content that has been encrypted and then distributed is imported into a network (such as a home network), the content can be bound to the network by having terminals on the network use the encrypted content without conversion and by converting the encryption of the encryption key for the content. Here, “converting the encryption” refers to a process where the encryption key for the content is re-encrypted by using an encryption key (hereinafter called a “network key”) that has been shared on the network in advance.
When a content that has been bound to the network is used, an appliance on the network that stores the network key can use the shared network key to decrypt the content encryption key that has been re-encrypted, and then use the decrypted content encryption key to decrypt the encrypted content.
On the other hand, an appliance that does not store the network key cannot decrypt the content encryption key and so is unable to decrypt the encrypted content.
With the content using systems described in the patent documents #1 to #3 listed below, limitations are imposed on the usage of contents as follows. Once a content has been bound to a network, the generation of copies of a content and the content encryption key on storage media, such as hard disk drives, on the network is unlimited for such data in a state where the data is bound to the network. However, the number of network appliances that may use (for example, reproduce) the content is limited.
Patent Document #1
Japanese Laid-Open Patent Application No. 11-331805
Patent Document #2
The specification of U.S. Pat. No. 5,878,135
Patent Document #3
The specification of U.S. Pat. No. 6,016,384
With the background art listed above, in reality, it is not possible to write a content outside the network with the content in a state where the content is not bound to the network. In spite of this, there are demands from users who wish to write (“move” or “export”) a content that has been bound to the network to an external storage medium such as a DVD-RAM, D-VHS, an SD (Secure Digital) Card, etc., and then use the content.
In this way, there is a problem in that conventional content using systems do not consider the writing of unbound contents, and thus are unable to satisfy the user demands described above.
With such conventional systems, even if it were possible to write a content in an unbound state, there would be the risk of it being possible to make an unlimited number of copies of the content onto storage media. This leads to problems regarding copyright protection for the content.
The present invention was conceived in view of the above problems with the background art, and has an object of providing a content using system that provides, for contents that have been bound to a network, a favorable balance between the conflicting aims of satisfying the above user demands and providing copyright protection.
Putting this another way, it is an object of the present invention to provide a content using system that can provide sufficient copyright protection while satisfying the demands of users regarding the writing of contents, which have been bound to a network, onto a storage medium.